Apart from admixtures of grit, communal and industrial wastewaters contain coarse, floating pollutants, putrescent substances, sludge-forming suspended solids, dissolved organic substances and bacteria. The undissolved substances are separated in the mechanical cleaning of wastewater. Coarse contaminants are retained by bar screens, the grit is separated out in the grit chamber, the sludge is separated out in the clarification tank or sludge settling tank. Primary clarification tanks serve to remove settleable solids from the wastewater. The dissolved, putrescible organic substances that are still present after the mechanical cleaning of wastewater are degraded in the biological cleaning of wastewater by the bacteria located in the wastewater.
The method described at the start has already been prior art for years and is the most commonly used method for the biological cleaning of wastewater in sewage treatment plants. In said method, the mostly communal wastewater is cleared of organic contaminants, i.e., cleaned, to the greatest possible extent by the metabolic activity of aerobic chemoorganoheterotrophic microorganisms, the so-called activated sludge. The method starts after the removal or separation of the coarse fractions, which are dewatered, separated, digested and incinerated. For communal wastewaters, this method ranks among the classic intensive treatment methods. The advantage is the general usability and the good cleaning action for wastewaters for reducing the levels of suspended solids, the chemical oxygen demand (COD), the biochemical oxygen demand (BOD5) and the nitrogen compounds (N).
Plants based on the activated-sludge method can be operated either continuously, i.e., in continuous operation (conventional activated-sludge plant), or discontinuously (SBR plant). Furthermore, there are also additionally so-called membrane activated-sludge plants, in which a membrane is used to remove the cleaned water from the sludge. What is common to all variants is that the water-suspended bacteria mass or biomass, which is also referred to as activated sludge, assumes the biological cleaning of the wastewater. To this end, each plant has at least one activated-sludge tank, in which the wastewater is admixed with the activated sludge and thus intensively contacted with the activated sludge.
Activated sludge means the biomass formed in the activated-sludge tank in the aerobic biological cleaning of wastewater as a result of the degradation of the wastewater ingredients. It substantially consists of microorganisms, such as, for example, bacteria, fungi, protozoa, EPS (extracellular polymeric substances) and further constituents. Microscopic studies provide evidence that the activated sludge is “activated” by bacteria and protozoa. Therefore, it is also called activated sludge. When used technically in the activated-sludge method, the activated sludge is generally present in the form of activated-sludge flocs, which contain, besides living and dead biomass, adsorbed and embedded organic compounds and minerals.
In the activated-sludge method, the degradation of the harmful substances in the wastewater by the activated sludge is followed by a separation of said sludge from the cleaned water in the so-called secondary clarification unit. The majority of the removed sludge is recycled into the activated-sludge tank as return sludge or recirculation sludge. This ensures that the activated-sludge concentration in the activated-sludge tank can be maintained. The activated flocs present in the return sludge renew the cleaning power of the activated material. The non-recirculated, smaller volume stream of the activated sludge is called excess sludge. The excess sludge is thus the part of the activated sludge which is drawn off and pumped into the sludge treatment unit in order to keep the desired biomass concentration constant. This removed biomass growth is generally fed together with the primary sludge to the sludge digestion unit and lastly to the sludge dewatering unit.
In the activated sludge, there is the formation under certain boundary conditions, especially in the case of activated-sludge plants having a low sludge load (<0.2 kg BOD5/(kg DS d)), of filamentous bacteria (bulking sludge), which can prevent the settling process in the secondary clarification tank or, in the case of an SBR plant, in the activated-sludge tank and lead to the output of activated sludge and thus severely impair the effluent quality. Over 65% of all activated-sludge plants have a bulking-sludge problem from time to time. The settling properties of the sludge are the crucial parameter in the assessment of an activated-sludge plant.
The most common filamentous bacteria are bacteria of the type Microthrix parvicella, Nocardia, type 021 N and others. Owing to the filamentous structure, these bacteria prevent the settling of the sludge, resulting in an inadequate separation of sludge water.
In many cases, this problem is handled by nonspecific measures, such as, for example, the addition of precipitants based on aluminum to the activated-sludge tank. However, it is not possible in many cases to improve the settling properties to the desired extent. In addition, the addition of these chemicals additionally produces sludge which must be subsequently disposed of.
In the case, too, of activated-sludge steps under very high load, for example adsorption steps, also called A steps (sludge load>2 kg BOD5/(kg DS d)), the major problem is that of achieving an easily settleable sludge. The settleability of the activated sludge in the A step and, accordingly, the settling and the recycling to the activated-sludge step that is under high load are the major challenge in the assessment and in the operation of such a plant.
The settling properties of the activated sludge determine the size of the secondary clarification unit (settling tank or secondary clarification tank). The more poorly the activated sludge settles, the larger the size of the secondary clarification unit must be. The better the sludge settles, the smaller the secondary clarification unit can be, the higher the activated-sludge concentration in the activated-sludge tank can be and, accordingly, the smaller the size of the required volume of the activated-sludge tank and of the secondary clarification tank can be. The sludge settling properties are crucial to the size of the entire activated-sludge plant and have a crucial impact on the construction costs of the activated-sludge plant.